Instrument and method for heating value measurement by stoichiometric combustion

ABSTRACT

A method and apparatus for determining the calorific value of a first combustible gas involves mixing the first combustible gas with a second combustible gas having a known calorific value and with a combustion supporting gas and burning the resulting mixture, detecting a property of the burning mixture indicative of whether the burning occurred at the stoichiometric point, adjusting the relative flow rates of the first and second combustible gases and the combustion supporting gas so that said burning occurs substantially at the stoichiometric point or at a selected offset from that point, and ascertaining the volume ratios of the gases at the adjusted flow rate to produce a value proportional to the overall calorific value of the mixture of the first and second combustible gases. Based on the foregoing, the contribution to the overall calorific value made by the second combustible gas having a known calorific value can be deleted to yield the calorific value of the first combustible gas.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to apparatus and methods for measuring theheating value of gaseous fuels and, more particularly, to apparatus andmethods for measuring the heating value of gaseous fuels having lowheating values.

2. History of the Prior Art

The calorific value of a combustible gas has been defined as thequantity of heat in British Thermal Units (BTU) which is released whenone standard cubic foot of gas is completely oxidized at a temperatureof 60° F. and any water produced by oxidation is in the liquid state.When the gas is a hydrocarbon or a mixture of hydrocarbons, theoxidation products of complete oxidation are carbon dioxide and water;and when one standard cubic foot of the gas is mixed with a sufficientquantity of oxygen at 60° F. to completely oxidize the gas, oxidation iscarried out and the products thereof, carbon dioxide and water, arecooled to 60° F. and all water is condensed to liquid state. The totalheat given off, including the heat transferred in cooling the productsand in condensing all of the water, is the calorific value of the gas.

Calorific value so defined is used extensively in industry as a measureof the quality of a gas or other fuel. If a gas is to be fed to aburner, the proper operation of the latter is often highly dependentupon calorific value whereby it is essential to control such valuewithin narrow limits. Accordingly, it is common practice for suppliersand users of combustible gas to monitor the calorific value thereof.

Since the calorific value of a combustible gas depends only on itschemical composition, that value can be determined by a completechemical analysis of the gas if the calorific value of each of itsconstituents is known. However, this method is time-consuming andimpractical for continuously monitoring the calorific value.

Many standard methods for measuring calorific value involve mixing andburning a known volume of a combustible gas with an excess ofoxygen-containing or combustion-supporting gas, transferring theresultant heat to a heat absorbing fluid and measuring the quantity ofheat transferred. The conditions for these operations would ideally bethe same as in the above definition of calorific value. Any deviationfrom these conditions will cause the resultant heat transferred perstandard cubic foot of combustible gas in the measurement to bedifferent from the calorific value.

In practice, it is difficult to maintain the rigid conditions requiredfor correct measurement. Part of this difficulty stems from the initialtemperatures of the combustible and oxygen-containing gases seldom, ifever, being 60° F. Also, the temperature of the combustion productsafter heat transfer is not 60° F. and, usually, is higher than theinitial temperature of the gases. The water produced is seldom condensedto the liquid state and the heat absorbing fluid never absorbs all ofthe heat transferred from the combustion products, since some heat isalways lost by radiation and conduction.

Each of the foregoing deviations is a source of measurement error andcorrection thereof requires complicated, expensive apparatus and, often,special environmental control.

In response to the above, methods of and means for measuring thecalorific value of combustible gases which do not depend upon measuringthe amount of heat released in combustion, which are not affected by theerrors discussed above, which are capable of continuous operation, andwhich are not affected by ambient temperature and other varyingenvironmental factors have been developed. One such method and means isdisclosed in U.S. Pat. No. 3,777,562 to William H. Clingman, Jr.Briefly, Clingman teaches burning a mixture of a combustible gas and acombustion supporting gas in one or more flames, monitoring thetemperature or temperatures of the burned gases, and adjusting thevolume ratio of the combustion-supporting gas to the combustible gas soas to maintain the temperature or the average of the temperatures atsubstantially maximum (i.e., at the stoichiometric point). Because, astaught by Clingman, the volume ratio of the gases which produces themaximum temperature (i.e., the volume ratio at the stoichiometric point)varies substantially directly with the calorific value of thecombustible gas, the calorific value of the combustible gas may bedetermined.

"Stoichiometric combustion" instruments, such as that invented byClingman and described above, are both accurate and rapid in operationwhen they can be employed. However, they can only be employed when thegas being analyzed will form a combustible mixture with air. In certainsituations, such as those involving flares, it is often necessary tomeasure gases with low heating values, e.g., 100-200 BTU/scF. Thesegases may not burn in a premixed flame (although they may burn in adiffusion fed flame, and are in that sense combustible); thus, theheating value of these gases may not be measured by the "stoichiometriccombustion" (e.g., Clingman) apparatus, instruments, and methodsdescribed above. Additionally, the "stoichiometric combustion"apparatus, instruments, and methods described above always have limitedranges. This is because there is a limited air-fuel ratio for which aflame will be stable in a given burner.

SUMMARY OF THE INVENTION

In accordance with the present invention, a method and apparatus fordetermining the calorific value of a first combustible gas involvesmixing the first combustible gas with a second combustible gas having aknown calorific value and with a combustion supporting gas and burningthe resulting mixture, detecting a property of the burning mixtureindicative of whether the burning occurred at the stoichiometric point,adjusting the relative flow rate of the first and second combustiblegases and the combustion supporting gas so that the burning occurssubstantially at the stoichiometric point or at a selected offset fromthat point, and ascertaining the volume ratios of the gases at saidadjusted flow rates to produce a value proportional to the overallcalorific value of the mixture of the first and second combustiblegases. Based on the foregoing, the contribution to the overall calorificvalue made by the second combustible gas having a known calorific valuecan be deleted from the volume ratios to yield a calorific value of thefirst combustible gas.

One instrument and method according to the present invention involvesperforming adjustment of flow rates by holding the flow rates of thefirst combustible and the combustion supporting gas substantially fixedand adjusting the flow rate of the second combustible gas. Anotherinstrument and method according to the present invention involvesperforming adjustment by mixing the two combustible gases together inselected ratio and adjusting the flow rate of the combustion supportinggas.

Accordingly, an object of the present invention is to provide anapparatus and method for measuring the heating value of a gas that isespecially suitable for use in control situations.

Another object of the present invention is to provide an apparatus andmethod capable of continuously measuring heating value over a wide rangee.g., 0-1800 BTU/scF.

Yet another object of the present invention is to provide a method andapparatus for measuring the heating values of gases (e.g., 100 BTU/scFmixture of methane with nitrogen) that would be, by themselves, notcombustible when premixed with air.

Other objects, advantages and novel features of the present inventionwill become apparent from the following detailed description of theinvention when considered in conjunction with the accompanying drawingwherein:

BRIEF DESCRIPTION OF THE DRAWING

The sole, FIG. 1, is a diagrammatic view illustrating a preferredembodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

A preferred method for measuring the heating value of a gas can be bestunderstood by reference to FIG. 1, wherein a suitable apparatus 10 isillustrated diagrammatically. For convenience, combustion-supporting oroxygen containing gas will be referred to as "dry air" or "air" with theunderstanding that any suitable gas is included within this designation.A gas with a known heating value will be referred to as a "combustiblegas" or "second combustible gas having a known calorific value". Propaneor methane would be suitable combustible gases; a number of other gases,well known to those skilled in the relevant art, would also be suitablecombustible gases. Finally, the gas to be measured by the apparatus ofthe present invention will be referred to by the term "sample gas" or"first combustible gas."

Basic elements of apparatus 10 are lines 12, 14, 16 for allowing flow ofdry air, combustible gas, and sample gas, respectively, into apparatus10. At point 18, lines 12, 14, and 16 converge; at this point, any gasesflowing through lines 12, 14 and 16 will be mixed.

Apparatus 10 also includes a means for burning, which is shown in FIG. 1as a flame using burner 20 having a top 22. Burner 20 has a flamesupporting grid 24 at its outlet and line 26, which is connected tomixing point 18, at its inlet.

Still further, apparatus 10 includes a means for determining when themixed gases input into burner 20 are essentially at their stoichiometricpoint. In FIG. 1, this means is shown as a thermocouple 28. Thermocouple28 is disposed within a large flame 30 which results when, in the caseof many carbon-containing fuels, the carbon monoxide combustion productburns and mixes with ambient air. In any event, as shown by referencenumeral 32, a large number of small flames or flamelets emanate fromgrid 24 upon burning of mixed gas(es) and air. Those skilled in the artknow that maximum flame temperature occurs essentially at thestoichiometric point of burning mixed gas and air; thus, those skilledin the art will readily appreciate that by serving as a means forindicating when flame 30 and/or flamelet 32 temperature is at itsmaximum, an input mixture of air and gas or air and gases is essentiallyat its stoichiometric point.

As an alternative means for determining the stoichiometric point, anoxygen meter 29 may be used instead of (or as a supplement to)thermocouple 28 in apparatus 10. Change in oxygen content of combustedgases is maximized for a fixed incremental change in mixture ratio. Thismaximum change occurs at the stoichiometric point. Accordingly, oxygenmeter 29 may be employed to determine the point at which oxygen contentof combusted gases is at a minimum: that point will be thestoichiometric point.

At this point, sufficient elements have been described to describeexercise of the method of the present invention.

Briefly, a preferred method of the present invention involvesdetermining, for a fixed air flow, the stoichiometric energy flow of acombustible gas, both with and without a measured volumetric flow ofsample gas entering the combustion zone. The difference between the twoenergy flows of the combustible gas divided by the known volumetric flowof sample gas is the heating value of the sample gas. In the practice ofsuch method it does not necessarily matter in what order or amount thethree elements of the mixture are combined so long as the first andsecond combinations reach stoichiometric points as described. Furtherdetails concerning the present invention are set forth below. Forclarity and convenience, in the ensuing description the combustible gaswill be referred to simply as propane and the following letters andsubscripts will be used as defined:

F_(s) =volumetric flow of the sample gas

F_(p) =volumetric flow of propane

E_(s) =energy flow of the sample gas

E_(p) =energy flow of propane

E_(a) =equivalent air energy flow

C_(p) =heating value of the propane

C_(s) =heating value of the sample gas

During operation of the present invention a fixed amount of dry air ismixed with propane and burned in burner 20. Via flow control 34 theamount of propane is adjusted until the mixture of dry and air propaneis essentially at the stoichiometric point. This can be accomplished bymaximizing flame temperature as detected by thermocouple 28.

After the propane/dry air mixture is at its stoichiometric point, samplegas is added to the gaseous mix. The stoichiometric combustion principleimplies that the total energy flow into burner 20 (i.e., the sum of thesample gas and propane energy flows) is approximately equal to theequivalent energy flow of air when it is in proportion to propane alone.The equivalent air energy flow, i.e., E_(a), can be determinedperiodically by titrating the constant air flow with propane underconditions of no sample gas flow.

In apparatus 10 the volumetric flows of the sample gas (F_(s)) andpropane (F_(p)) may be measured using conventional flow meters 36 and 38respectively, or by another other suitable, known method for measuringflow. As previously indicated

    E.sub.a =E.sub.p +E.sub.s.                                 (1)

Additionally

    E.sub.p =F.sub.p ·C.sub.p.                        (2)

and

    E.sub.s =F.sub.s ·C.sub.s.                        (3)

Substituting F_(p) ·C_(p) for E_(p) in equation (1) leads to

    E.sub.a =F.sub.p ·C.sub.p +E.sub.s.               (4)

Rearranging terms leads to

    E.sub.s =E.sub.a -F.sub.p ·C.sub.p.               (5)

Substituting F_(s) ·C_(s) for E_(s) in equation (5) leads to

    F.sub.s ·C.sub.s =E.sub.a -F.sub.p ·C.sub.p.(6)

Dividing both sides of the equation by F_(s) results in

    C.sub.s =(E.sub.a -F.sub.p ·C.sub.p)/F.sub.s.     (7)

Equation (7) provides a basis for calculating heating value. C_(p) isknown from the quality of the propane being used; F_(p) and F_(s) areeasily measured using flowmeters 38, 36; and E_(a) is equivalent toF_(p) '·C_(p) where F_(p) ' is the volumetric propane flow with nosample gas flow.

Referring again to FIG. 1, it may be seen that a preferred embodiment ofthe present invention may also include a computer 42 for apparatus 10control. Signals from thermocouple 28 (and/or oxygen meter 29)indicating flame temperature (and/or indicating the oxygen content ofthe combusted gases) may be fed through an amplifier 40 (or perhaps evendirectly) to computer 42, which may be programmed to maximize thattemperature (and/or maximize rate of oxygen content change) byexercising control on propane flow via flow control 34. Computer 42 mayalso be enabled to exercise some control over air and sample gas flowsvia flow controls 44 and 46, respectively.

The upstream air flow and sample gas flow are set so that the two flowsproduce a mixture that is always lean. The mixture must be lean for thehighest sample gas heating value that is to be measured and for alllower heating values. The mixture must also be lean for pure hydrogen.This has a heating value of only about 330 BTU/scF compared to 1012BTU/scF for methane. Because of the low density of the hydrogen,however, much more of this gas will flow into the combustion zone thanfor a hydrocarbon fuel. This is particularly true where the latter isdiluted with a high density gas such as carbon dioxide.

A typical setting for maximum range might be 1200-1800 BTU/scF for usein a refinery or chemical processing plant. In some applications,however, the maximum range would be lower as further described below. Inthe field apparatus 10 can produce a continuous measurement of C_(s)that can be used for control purposes. If the sample gas is a mixture ofinert gas and paraffin hydrocarbons then the stoichiometric combustiontheory implies that the measurement will be exact. If the samplecontains hydrogen then there will be an error up to 80 BTU/scF for purehydrogen. Provision can be made in the instrument (e.g., in a program incomputer 42) to partially correct for such errors if the compositionrange is known in advance. In any case, for most control applicationsthe accuracy will be sufficient.

Numerous modifications and variations are possible in light of the aboveteachings. For example, as discussed in the Summary of the Inventionsection above, an alternate embodiment of the invention could involveperforming adjustment by mixing two combustible gases together andadjusting flow rate of a combustion supporting gas. Other embodimentsare possible. In embodiments of the present invention all three elementscould be combined at the same time and the respective flowrates adjustedin order to burn the mixture at its stoichiometric point so that thecalorific value of the sample gas can be determined. It is essentialonly that values of sufficient parameters discussed herein be derived toallow calculation of the ultimate calorific value of the sample gas. Itis therefore to be understood that within the scope of the appendedclaims, the invention may be practiced otherwise than as specificallydescribed.

We claim:
 1. A method for determining the calorific value of a firstcombustible gas comprising:mixing said first combustible gas with asecond combustible gas having a known calorific value and with acombustion supporting gas and burning the resulting mixture; detecting aproperty of the burned mixture indicative of whether said burningoccurred at essentially the stochiometric point; adjusting the relativeflow rates of said first and second combustible gases and saidcombustion supporting gas so that said burning occurs substantially atsaid stoichiometric point or at a selected offset from said point;ascertaining the volume ratios of said gases at said adjusted flow ratesto produce a value proportional to the overall calorific value of saidmixture of said first and second combustible gases; and deleting fromsaid ratios the contribution to said overall calorific value made bysaid second combustible gas having a known calorific value, to yield thecalorific value of said first combustible gas.
 2. A method in accordancewith claim 1 in which said indicative property is temperature, which issubstantially maximized at the stoichiometric point.
 3. A method inaccordance with claim 1 in which said indicative property is change ofoxygen content in the combusted gases with fixed changes in the mixtureratio, which change in oxygen content is substantially maximized at thestoichiometric point.
 4. A method in accordance with claim 1 in whichsaid adjustment of flow rates is performed by holding the flow rates ofsaid first combustible gas and said combustion supporting gassubstantially fixed and adjusting the flow rate of said secondcombustible gas.
 5. A method in accordance with claim 1 in which saidadjustment is performed by first mixing said two combustible gasestogether in selected ratio and adjusting the flow rate of said two-gasmixture with respect to the flow rate of said combustion supporting gas.6. An apparatus for determining the calorific value of a firstcombustible gas comprising:means for mixing said first combustible gaswith a second combustible gas having a known calorific value and with acombustion supporting gas and burning the resulting mixture; means fordetecting a property of the burned mixture indicative of whether saidburning occurred at the stoichiometric point; means for adjusting therelative flow rates of said first and second combustible gases and saidcombustion supporting gas so that said burning occurs substantially atsaid stoichiometric point or at a selected offset from said point; andmeans for ascertaining the volume ratios of said gases at said adjustedflow rates to produce a value proportional to the overall calorificvalue of said mixture of said first and second combustible gases;whereby the contribution to said overall calorific value made by saidsecond combustible gas having a known calorific value can be deletedfrom said ratios to yield a calorific value of said first combustiblegas.
 7. An apparatus as recited in claim 6, wherein said indicativeproperty is temperature, which is substantially maximized at thestoichiometric point.
 8. An apparatus as recited in claim 6, whereinsaid indicative property is change of oxygen content in the combustedgases with changes in the mixture ratio, which change in oxygen contentis substantially maximized at the stoichiometric point.
 9. An apparatusas recited in claim 6, wherein said adjustment of flow rates isperformed by holding the flow rates of said first combustible gas andsaid combustion supporting gas substantially fixed and adjusting theflow rate of said second combustible gas.
 10. An apparatus as recited inclaim 6, wherein said adjustment is performed by first mixing said twocombustible gases together in selected ratio and adjusting the flow rateof said two-gas mixture with respect to the flow rate of said combustionsupporting gas.
 11. An apparatus for determining the heating value perunit volume of a gas, said apparatus comprising:(a) means for allowing aknown volume flow of said gas; and (b) means for determining heatingvalue of said gas at said known volume flow, said means for determiningheating value comprising:(i) means for allowing flow of a gas with aknown heating value per unit volume; (ii) means for adjusting flow ofsaid gas with a known heating value; (iii) means for mixing said gaswith said gas with a known heating value; (iv) means for allowing flowof a combustion supporting gas; (v) means for mixing said combustionsupporting gas and said gas with a known heating value; and (vi) meansfor burning gas; whereby said gas with a known heating value can beadjusted in flow to a set point, said gas can be mixed with said gaswith a known heating value, and said gas with a known heating value canbe readjusted in flow to said set point.
 12. An apparatus as recited inclaim 11, wherein said gas with a known heating value is mixed with saidcombustion supporting gas to form a first mixture, wherein said firstmixture is applied to said burning means, and wherein flow of said gaswith a known heating value is adjusted so that said first mixture isessentially at the stoichiometric point.
 13. An apparatus as recited inclaim 12, wherein said gas is mixed with said first mixture to form asecond mixture, wherein said second mixture is applied to said burningmeans and wherein flow of said gas with a known heating value isadjusted so that said second mixture is essentially at thestoichiometric point.
 14. An apparatus as recited in claim 13, whereinsaid means for determining heating value further comprises:(vii) meansfor determining rate of flow of said gas with a known heatingvalue;whereby a first energy flow rate when said first mixture isessentially at the stoichiometric point can be determined, a secondenergy flow rate when said mixture is essentially at the stoichiometricpoint can be determined, and the difference between said first energyflow rate and said second energy flow rate can be calculated.